This invention relates to a method and apparatus for processing a substrate, for example, for carrying out vapor deposition, wafer transfer, or other processes on a semiconductor substrate.
Chemical (CVD) or physical (PVD) vapor deposition is usually carried out using a cluster tool. Such a tool is typically of modular design and comprises degas stations, which permit the removal of gases from the silicon wafers being processed; transfer modules, which isolate the various process modules and allow transfer of the wafers therebetween; etch process modules, which use inert gases to sputter clean oxides; physical vapor deposition (PVD) process modules, which use inert gases to sputter deposit various compounds, such as thin films; reactive PVD process modules, which use mixtures of inert gases and reactive gases in order to reactively sputter deposit various compounds, as thin films; chemical vapor deposition (CVD) process modules, which use relatively low vapor pressure reactive gases in order to reactively deposit various compounds, as thin films, via a chemical reaction at low pressure; and load locks, which isolate the internal components of the cluster tools from the ambient air.
Examples of commercially available cluster tools are Varian's M2000/8 and M2i (California, USA), Applied Materials' Endura HP (California, USA), Novellus' Concept Two Altus (California, USA), Materials Research Corporation's Eclipse Star and Galaxy (NY, USA), as well as Anelva's 1061 (Japan).
These cluster tools all use expensive vacuum load locks, special degas stations, expensive ultra high vacuum deposition modules, expensive dry pumps, large and expensive ultra-high vacuum cryogenic pumps, and special lamps and heaters. They require frequent regeneration and many other special components in order to obtain an ultra-high vacuum, to maintain ultra-high purity gas distribution and to prevent cross contamination.
Argon (or any other inert gas) may be used in the etch process modules and in the PVD process modules. A mixture of argon (or any other inert gas) and nitrogen (or any other reactive gas) is typically used in the reactive PVD process modules. Various chemicals are used in the CVD process modules.
The purity of the gases used in the PVD, reactive PVD and CVD process modules is very important in order to ensure controlled film properties. The gases used in the reactive PVD and CVD process modules constitute impurities for the etch and PVD process modules. Cross contamination between process modules must therefore be avoided and is the basic design consideration in current state of the art deposition cluster tools.
The carrying out of both CVD and PVD in the same cluster tool is particularly demanding of the equipment because of the difficulty in preventing cross contamination between the CVD and PVD processing modules. Very expensive large pumps are required to achieve the necessary pumping rates.
Deposition cluster tools use very large turbomolecular pumps or cryogenic pumps, complex gas distribution sequences, and have very long delays between the end of a process in a module and the opening of isolation valves of that module in order to prevent cross contamination between the various process modules.
Nitrogen, carbon monoxide, water vapor and hydrogen are some of the impurities that must be removed from argon (or from any other inert gas) before during, and after the etch or the PVD. As a result, expensive vacuum load locks are used to create the vacuum and ensure that the transfer modules and the process modules are well isolated from air which contains the impurities. Special degas modules are used to remove any trace of such impurities from the wafers' porous materials.
Intensive ultra-high vacuum modules, almost all of which have metal vacuum seals, with vacuum baked O-rings, polished sealing surfaces (which eliminate the need for vacuum grease), and very high purity, very dense, and very expensive alumina ceramics are used to achieve ultra-high vacuum. Dry pumps in addition to very expensive ultra-high vacuum cryogenic pumps are used to achieve ultra-high vacuum. Special in-situ ultraviolet or in-situ infrared lamps with external heaters are used as bake-out devices in order to heat the walls of the module and to facilitate the liberation and the pumping of the adsorbed/absorbed moisture which results in the opening of the modules to ambient air for a few minutes.
The typical waiting time of 10 to 12 hours to achieve ultra-high vacuum represents a very important downtime and is limited by the liberation of the adsorbed/absorbed moisture and by the pumping speed of the cryogenic pumps. In situ plasmas are used to help in the liberation of the adsorbed/absorbed moisture. These plasmas cause the dissociation of moisture into hydrogen and oxygen and since hydrogen is difficult to pump with cryogenic pumps, there is a tendency to accumulate hydrogen as residual gas and to limit the base pressure of the module. This situation results in the need for a regeneration of the cryogenic pump in order to re-activate the coconut charcoal and the hydrogen pumping with the cryogenic pump. Although a long regeneration cycle, in the order of 3 to 4 hours, improves the situation, the hydrogen equilibrium pressure remains the limiting factor and controls the vacuum performance (i.e. base pressure) of the module.
Even these complex, expensive state of the art deposition cluster tools can still only deliver an imperfect vacuum performance, gas impurity and contamination control for more demanding applications such as aluminum plugs, which require high temperature diffusion of aluminum alloys to fill small diameter contacts and vias, as well as for mixed PVD/CVD applications.
The aluminum plug process carried out in such cluster tools is very unstable because the high temperature self-diffusion of aluminum alloys is highly affected by the residual impurity levels present. Problems observed are: the filling of contacts and vias of small diameter and high aspect ratio; the roughness of the top surface of self-diffused aluminum alloys; and the photolithography of the obtained aluminum alloys all become erratic because the control of the residual gaseous impurity levels is still too marginal. This undesirable situation is the result of the marginal pumping speed of the large and expensive cryogenic pumps.
Electromigration and stress voiding of the aluminum interconnects is very sensitive to trace levels of gaseous impurities. Existing deposition cluster tools are only marginally able to guarantee repeatable reliability of the interconnects. The marginal pumping speed of the expensive cryogenic pumps is at the root of the problem.
Integration of PVD and of CVD in one deposition duster tool is difficult. The limited pumping speed of the cryogenic pumps makes the prevention of cross contamination from the low vapor pressure chemicals used in the CVD process modules difficult.
The vibration of the large cryogenic pumps, the frequent cycling of the process gases (which results in local turbulence and abrupt pressure surges), and the frequent opening and closing of isolation valves cause particle contamination in the deposited films.
These factors together result in a very expensive, very complex, and very sophisticated deposition cluster tool with a very extensive set of spare parts. Despite this, unscheduled downtime still results from the failure of these sophisticated components and from the failure of the associated sophisticated software. Scheduled downtime results from the waiting time needed to achieve ultra-high vacuum conditions and from the regeneration time of the various cryogenic pumps.
Sophisticated software is needed to control the sophisticated hardware. Furthermore, the necessarily very large system uses a lot of expensive clean room space.
Most of these problems are associated with the need for sophisticated hardware, which ensures that the following five requirements are met: isolation from ambient air, suitable wafer degassing, ultra-high vacuum, ultra-high purity gas delivery, and prevention of cross contamination.
An object of the invention is to alleviate the afore-mentioned problems.